Plotting of Mannual Issodose line in different Cancer cases in Radiotherapy treatment plan and it is compared with Varian Eclipse Treatment Planning System.
Comparision done in 2019 at Tata Memorial Hospital, Parel, Mumbai.
6. Isodose shift method
Used for Manually correction
of Isodose lines.
Along each grid lines(1 cm),
Isodose line is shifted by an
amount:
k x h
7. h (tissue deficit or tissue excess) & “k” a factor
less than 1 and is given in the following Table:
Isodose position along the grid are joined after
correction.
11. Gf
t
t
where
Gf
Gf
t
t
t
F
factor
Shift
2
..
..
2
)
(
_ '
'
FS=10x10cm2 ,SSD(f)=100cm
and for depth=7cm G=0.068
then
Shift factor:
71
.
0
100
068
.
0
2
100
068
.
0
2
Gf
Gf
F
Simillarly, corresponding values for
5x5cm2 and 20x20cm2 is 0.74 and
0.68.
(Giessen PH.A method of calculating the isodose shift in correcting for oblique incidence in radiotherapy.Br J
Radiol. 1973;46:978.)
12. • K = f(gap,radiation energy,SSD,field area and depth in absorbing medium)
• When gap(t) is small compared to SSD , then k is independent of gap.
• Example:If SSD is 50cm then k is not independent of gap
• But it is dependent on other factors.
• For depths below about 7cm, the factor is sensibly constant.
• It is shown that a single value of the factor may be used for all fields and
depths(for given radiation energy and SSD) with an error up to 1% to 2%.
(Giessen PH.A method of calculating the isodose shift in correcting for oblique incidence in radiotherapy.Br
J Radiol. 1973;46:978.)
13. Effective SSD method
It include correction of PDD with the help of ISF.
Here isodose lines are shift by an amount h (tissue deficit or
tissue excess)
14. Here h is tissue deficit
As we are interested to find PDD at
point A as shown in the figure.
15. Tissue-air ratio method
Correction of PDD done with help of TAR/TMR
actual
corrected PDD
CF
PDD *
)
(
)
(
h
d
TMR
d
TMR
CF
,Here d and h are in cm.
17. Isodose shift method
Used for Manually correction of Isodose lines.
Proposed by Greene, Steward and Sundblom.
Shift are done by amount equal to n(emperically obtained
value) times the thickness of inhomogeniety.
It is measured along the line parallel to the central axis and
passing to the point of interest.
The shift is away from skin for lung and air cavity and
opposite in case of bone.
18.
19. Small bulges are due to inoization chamber of
0.6 mm diameter fixed at position R.
Measured with air filled hole where the isodose
curve moved distally by 0.6 times the thickness
of air gap.
(Greene D, Stewart Jr. Isodose curves in non_uniform Phantoms.Br J Radiol.1965;38:378.)
20. Measured with cork
(simulating Lung density )
where the isodose curve
moved distally by 0.4 times the
thickness of cork/ Lung.
(Greene D, Stewart Jr. Isodose curves in non_uniform Phantoms.Br J Radiol.1965;38:378.)
21. • The value of n is given by the following table:
• It is seen that the factor depends on the properties of material of different
densities and independent of field size.
• Error percentage is about 2% when the above emperical rules is compared
with measured Isodose lines .
22. Other correction factors
• Tissue_Air Ratio Method.
• Power Law Tissue_Air Ratio Method.
• Equivalent Tissue_Air Ratio Method.
25. • First take a contour of a patient CT
image with possible accuracy on a
Butterpaper.
• Take Standard Isodose Chart
• Place Contoured Butterpaper over
isodose chart
26. • Adjust the isodose line chart with the surface of body
contour.
• Draw isodose line coressponding to chart for
number of fields required.
• Apply correction factor whenever required.
69. Treatment time calculation
• Fraction size=725cGy
• Field size=8 X 8cm2
FIELD EFS
(cm2)
DEPTH
(cm)
PDD (%) OUTPUT
(cGy/min)
TT(min)
AP 8 10 54.8 55.18 5.99
PA 8 12 47.2 55.18 6.96
RL 8 18 30.2 55.18 10.87
LL 8 18 30.2 55.18 10.87
70. Time required for physicist
Procedure Time(Approx)
Four Fields(Grid) 4Hr
71. Conclusion
• Time consuming.
• Grid method shifts towards more accuracy.
• Grid method gives max dose recieving by OAR along
with PTV coverage.
72. References
• KHAN'S(The physics of radiation therapy) by Faiz M. Khan and John
P.Gibbons-5th Edition published by Wolters Kluwer(2014).
• Giessen PH.A method of calculating the isodose shift in correcting for
oblique incidence in radiotherapy.Br J Radiol. 1973;46:978.
• Greene D, Stewart Jr. Isodose curves in non_uniform Phantoms.Br J
Radiol.1965;38:378.